This invention relates to the treatment of liquids and more particularly to improved methods of upflow regeneration of ion exchange resins.
Ion exchange resins used to remove unwanted ions from liquids eventually become exhausted or spent and must have their liquid treating properties restored by regenerating them with chemical solutions that replace the unwanted ions with exchangeable ions. When liquid is treated by flowing it downwardly through a bed of ion exchange resin particles, it is often advantageous to regenerate the particles by flowing the regenerant solution upwardly in the opposite direction through the bed. This contacts the cleanest, least exhausted resin particles at the bottom of the bed with full strength regenerant solution. Such full strength solution more completely regenerates the particles at the bottom of the bed and thus produces a relatively efficient scrubber zone for polishing the liquid as it leaves the bed by removing ions at relatively low concentrations. The partially spent regenerant solution passing through the resin particles at the upper part of the bed still has enough strength to remove unwanted ions therefrom, and this increases the regenerating efficiency of the regenerant solution. However, the upwardly flowing regenerant solution tends to expand and upset the resin bed; this lowers regeneration efficiency which is highest when the bed is in a compact state and the resin particles are in contact with each other. Also, during upflow regeneration, channeling of the regenerant flow can cause regenerant to short circuit portions of the bed; this can prevent complete or efficient regeneration of the bed.
Prior attempts that have been made to prevent expansion of an upwardly regenerated resin bed have had serious disadvantages. The use of air pressure to block bed expansion requires expensive air compressors and controls, and also a six inch layer of inert material on top of the resin bed. Downwardly flowing raw water has been used to block upward bed expansion, but this results in a large volume of waste liquid that must be neutralized before it can be disposed of. And precipitation of calcium sulfate can occur when sulfuric acid is used to regenerate a layered bed of strong and weak acid cation resins.